It is known that single-junction photovoltaic cells are not capable of efficiently exploiting all the solar radiation. Their efficiency, in fact, is maximum only within a certain spectrum range which comprises a part of visible radiation and a part of infrared radiation.
Spectrum converter materials capable of capturing solar radiation outside the optimal spectral range and og converting it to effective radiation, can be used for enhancing the performance of photovoltaic cells. Furthermore, luminescent solar concentrators (LSC) can be produced with these materials, which allow a further increase in the production of current of photovoltaic cells.
Said luminescent solar concentrators (LSC) generally consist of large sheets of material transparent to solar radiation, in which fluorescent substances are dispersed, or chemically bound to said material, which act as spectrum converters. Due to the effect of the optical phenomenon of total reflection, the radiation emitted by the fluorescent molecules is “driven” towards the thin edges of the sheet where it is concentrated on photovoltaic cells or solar cells positioned therein. In this way, large surfaces of low-cost materials (photoluminescent sheets) can be used for concentrating the light on small surfaces of high-cost materials (photovoltaic cells or solar cells).
A fluorescent compound should have numerous characteristics for being advantageously used in the construction of luminescent solar concentrators (LSC) and these are not always mutually compatible.
First of all, the frequency of the radiation emitted by fluorescence must correspond to an energy higher than the threshold value below which the semiconductor, representing the core of the photovoltaic cell, is no longer able to work.
Secondly, the absorption spectrum of the fluorescent compound should be as extensive as possible, so as to absorb most of the including solar radiation and then to re-emit it at the desired frequency.
It is also desirable that the absorption of the solar radiation be extremely intense, so that the fluorescent compound can exert its function at the lowest possible concentrations, avoiding the use of huge quantities.
Furthermore, the absorption process of solar radiation and its subsequent re-emission at lower frequencies, must take place with the highest possible efficiency, minimizing the so-called non-radiative losses, often collectively indicated with the term “thermalization”: the efficiency of the process is measured by its quantum yield.
Finally, the absorption and emission frequencies must be as diverse as possible, as, otherwise, the radiation emitted by a molecule of the fluorescent compound would be absorbed and at least partially diffused by the adjacent molecules. This phenomenon, normally called self-absorption, inevitably leads to a significant loss in efficiency. The difference between the frequencies of the peak with the lower frequency of the absorption spectrum and the peak of the radiation emitted, is normally indicated as Stokes shift and measured as nm (it is not the difference between the two frequencies that is measured, but the difference between the two wavelengths which correspond to them). High Stokes shifts are absolutely necessary for obtaining high efficiencies of luminescent solar concentrators (LSC), bearing in mind the already mentioned necessity that the frequency of the radiation emitted corresponds to an energy higher than the threshold value below which the photovoltaic cell cannot function.
It is known that some benzothiadiazole compounds, in particular 4,7-di(thien-2′-yl)-2,1,3-benzothiadiazole (DTB), are fluorescent compounds which can be used in the construction of luminescent solar concentrators (LSC). Compounds of this type are described in Italian patent application MI 2009 A 001796 in the name of the Applicant.
4,7-di(thien-2′-yl)-2,1,3-benzothiadiazole (DTB) is characterized by an emission centred around 579 nm, which corresponds to an energy well above the minimum threshold value for the functioning of photovoltaic cells, said threshold corresponding for example to a wavelength of about 1100 nm for the most widely-used cells, based on silicon. Furthermore, its absorption of the light radiation is intense and extends over a relatively wide range of wavelengths, indicatively ranging from 550 nm (green radiation wavelength) to ultraviolet. Finally, 4,7-di(thien-2′-yl)-2,1,3-benzothiadiazole (DTB) has a Stokes shift in dichloromethane solution, equal to 133 nm, well above that of most of the commercial products so far proposed for use in luminescent solar concentrators.
For these reasons, the use of 4,7-di(thien-2′-yl)-2,1,3-benzothiadiazole (DTB) has enabled the production of high-quality luminescent solar concentrators (LSC).
Although 4,7-di(thien-2′-yl)-2,1,3-benzothiadi-azole (DTB) absorbs a significant part of the solar spectrum, however, it has a modest absorption in its higher wavelength regions, corresponding to yellow and red radiations which cannot therefore be converted into other radiations more effectively exploited by the photovoltaic cell. For this reason, it is desirable to avail of fluorescent compounds having a more red-shifted absorption spectrum.
The Applicant has therefore considered the problem of finding compounds having a more red-shifted absorption spectrum.